1. Field of the Invention
The present invention relates to a method of inhibiting the corrosion of metallic surfaces of water-carrying systems, and to compositions for use in such a method, particularly where the water of the system is oxygen-bearing. More particularly, the present invention relates to the use of compositions comprising a combination of polyphosphate and polymaleic anhydride or amine adducts thereof, and optionally zinc, to inhibit the corrosion of metallic surfaces of water-carrying systems.
The term "aqueous", as used herein, is intended to describe water in any physical state and to include water in which is dissolved or dispersed any substance, for example, inorganic salts in brine or seawater.
The term "metallic", as used herein, is intended to include metallic and metal-containing materials comprising ferrous, non-ferrous or alloy metal compositions.
Polymaleic anhydride, as used herein, is intended to include hydrolyzed polymaleic anhydride, which is essentially polymaleic acid. Under most ambient conditions, such hydrolysis to the acid form will take place.
Corrosion of the metallic surfaces of a water-carrying system consists of the destruction of the metal by chemical or electrochemical reaction of the metal with its immediate environment.
Where the corrosion is electrochemical in nature, a transfer or exchange of electrons is necessary for the corrosion reaction to proceed. When corrosion of the metal takes place, two partial electrochemical processes occur, and must occur, simultaneously. There is an anodic oxidation reaction in which metal ions go into solution, leaving behind electrons; and a cathodic reduction reaction in which species in solution are reduced by consuming the electrons produced by the anodic reaction. Where the metal is ferrous or ferrous-containing, and the water system contains oxygen, these two processes may be illustrated by the following equations: EQU Anodic oxidation: Fe.fwdarw.Fe.sup.++ +2e.sup.- EQU Cathodic reduction: 2H.sub.2 O+O.sub.2 +4e.sup.- .fwdarw.4OH.sup.-
The two ionic reaction products, ferrous ion and hydroxyl ion, combine to form ferrous hydroxide, Fe(OH).sub.2, which is then oxidized to form rust, ferric hydroxide, Fe(OH).sub.3. For ferrous or ferrous-containing as well as other metals in water systems, the principal factors influencing the corrosion process are the characteristics of the water of the system, the rate of water flow, the temperature of the system and the contact of dissimilar metals in the system. The variable characteristics of the water which determine its corrosiveness are its dissolved oxygen concentration, carbon dioxide content, pH and concentration of dissolved solids.
The presence of oxygen dissolved in the water of a system is primarily the result of contact of the water with the atmosphere. The oxygen solubility in water is temperature and pressure dependent, with an increase in pressure increasing solubility, and with an increase in temperature lowering the oxygen solubility.
Corrosion produced by the presence of oxygen in the water of a system can take place in the form of small pits or depressions besides general metal loss. As the corrosive process continues, these pits or depressions increase in area and depth and a nodule of corrosion products is formed. The corrosive attack is more severe when taking place in the form of pits or depressions since this permits deeper penetration of the metal and more rapid failure at these points.